As broadband transmission in mobile communication and IP communication has become the norm and services in such communications have diversified, high sound quality of and higher-fidelity speech communication is demanded. For example, from now on, hands free speech communication in a video telephone service, speech communication in video conferencing, multi-point speech communication where a number of callers hold a conversation simultaneously at a number of different locations and speech communication capable of transmitting the sound environment of the surroundings without losing high-fidelity will be expected to be demanded. In this case, it is preferred to implement speech communication by stereo speech which has higher-fidelity than using a monaural signal, is capable of recognizing positions where a number of callers are talking. To implement speech communication using a stereo signal, stereo speech encoding is essential.
Further, to implement traffic control and multicast communication in speech data communication over an IP network, speech encoding employing a scalable configuration is preferred. A scalable configuration includes a configuration capable of decoding speech data even from partial coded data at the receiving side.
As a result, even when encoding and transmitting stereo speech, it is preferable to implement encoding employing a monaural-stereo scalable configuration where it is possible to select decoding a stereo signal and decoding a monaural signal using part of coded data at the receiving side.
A monaural signal is generated from a stereo input signal in speech coding employing a monaural-stereo scalable configuration. For example, a method for generating monaural signals, includes averaging both channel (referred to as “ch” later) signals of a stereo signal and obtaining a monaural signal (see Non-Patent Document 1).
Non-patent document 1:
ISO/IEC 14496-3, “Information Technology-Coding of audio-visual objects-Part 3: Audio”, subpart-4, 4.B.14 Scalable AAC with core coder, pp. 304-305, December 2001.